Sheet-like article and method for making the same

ABSTRACT

A sheet-like article including fibers and carbon nanotubes and/or carbon nanohorns adhering to the surface of the fibers in a uniformly dispersed state without agglomeration to form a network structure on the fibers. The sheet-like article is preferably made by converting the fibers and a dispersion of the carbon nanotubes and/or carbon nanohorns by a wet papermaking process. To use cellulose fibers as main fibers provides a good sheet-like article.

TECHNICAL FIELD

This invention relaters to a sheet-like article that is fabricated offibers having carbon nanotubes and/or carbon nanohorns adhering theretoand is useful as an electrically conductive material, an electromagneticshielding material, an electromagnetic absorbing material, a microwaveheating element, an electrode, an ultrafine filter, a sheet heatingelement, a catalyst carrier, and so on. The invention also relates to amethod for producing the sheet-like article.

BACKGROUND ART

Carbon nanotubes (CNTs) and carbon nanohorns (CNHs) exhibit very highelectrical conductivity based on their unique structure and arereceiving attention for use as an electrically conductive material, anelectromagnetic shielding material, an electromagnetic absorbingmaterial, a microwave heating element, an electrode, an ultrafinefilter, a sheet heating element, and so on. Sheet materials containingCNTs or CNHs have been studied for these uses.

For example, patent document 1 (see below) proposes a sheet composed ofpulp and carbon fibers having a diameter of 0.01 to 4 μm and an aspectratio of 2 to 100,000. The sheet is produced by a method including thestep of mixing pulp and the carbon fiber. Therefore, the carbon fibersform agglomerates even though they are subjected to a treatment forimproving dispersibility, such as oxidation, coating, or grafting andfails to form a carbon fiber network on the cellulose fibers. Becausethe connections between carbon fibers are reduced due to the formationof carbon fiber agglomerates, it has been difficult to achieve the bestperformance characteristics of the carbon fibers, such as electricalconductivity, electromagnetic shielding characteristics, and microwaveabsorbing characteristics.

Patent document 2 (see below) discloses paper containing carbon fibrilagglomerates, in which carbon fibrils are intertwined with cellulosefibers. However, since the carbon fibrils exist in the form ofagglomerates, they do not form a sufficient network in the sheet, whichmakes it difficult to obtain the full performance of carbon fibrils, asis the case with the sheet of patent document 1.

Patent document 3 (see below) proposes a method for covering naturalfibers with CNTs by immersing natural fibers in a mixture of CNTs, asurfactant, and distilled water. This method, however, fails to achievesufficient covering of the cellulose fibers with CNTs. Moreover, naturalfibers containing a surfactant are not easily entangled with each otherso that, while usable as yarn for woven fabric, they are difficult to beformed into a sheet by a wet papermaking technique. Because the CNTs arecoated with the surfactant and are therefore inhibited from coming intocontact with each other, electron motion is interfered with so thatsufficient electrical conductivity is not provided.

Patent document 4 (see below) teaches a method for covering naturalfibers with CNTs by treating natural fibers with a dispersion of CNTs ina chemical plating bath. According to the method, natural fibers arecovered with CNTs via a metal, such as nickel. As a result, theperformance characteristics essentially possessed by CNTs, such aselectromagnetic shielding characteristics, microwave absorbingcharacteristics, and planarly heat generating properties, are not fullyexhibited. Although natural fibers treated with a chemical plating bathare usable as yarn for woven fabric as stated above, it is impossible toform them into a sheet by a wet papermaking technique.

Patent documents 5 and 6 (see below) disclose a method for dispersingCNTs and a sheet made by the method. The sheet is made solely of CNTsand is therefore very expensive and unable to retain the strength as asheet. In addition, the CNTs have a surfactant adhering thereto. Thenetwork formed of such CNTs does not sufficiently exhibit theperformance, such as electrical conductivity, because the surfactantinterferes with electrical conduction.

Patent document 7 (see below) proposes paper containing CNTs and amethod for producing the paper. However, the CNT fixing techniquedisclosed therein allows CNTs to agglomerate in the step of preparing apulp slurry, resulting in a failure to form a satisfactory network ofCNTs in the sheet. Therefore, it is difficult to make the full advantageof the electrical conductivity of CNTs, and it is necessary to add alarge quantity of CNTs to increase the conductivity of the sheet.

Thus, it has been difficult with conventional technology to form a CNT-and/or CNH-containing sheet having very effective electricalcharacteristics suited for use as an electrically conductive material,an electromagnetic shield, a microwave heating element, an electrode,and so forth.

Patent document 1: JP 63-288298APatent document 2: JP 7-97789APatent document 3: JP 2005-256221APatent document 4: JP 2005-256222APatent document 5: JP 2007-39623APatent document 6: JP 2007-63107APatent document 7: WO08/069,287

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a sheet-like article havingexcellent electrical characteristics and heat generating characteristicsand suitable for use as an electrically conductive (hereinafter simplyreferred to as “conductive”) material, an electromagnetic shieldingmaterial, an electromagnetic absorbing material, a microwave heatingelement, an electrode, and so on.

The invention provides a sheet-like article including fibers and a largenumber of CNTs and/or CNHs adhering to the fibers. The CNTs and/or CNHsadhere to the surface of the fibers in an uniformly dispersed statewithout agglomeration thereby to form a network structure on the fibers.

The invention provides an embodiment of the sheet-like article, which isobtained by a wet papermaking process using a dispersion of the CNTsand/or CNHs and the fibers.

The invention provides an embodiment of the sheet-like article, which isobtained by impregnating a fiber aggregate free from the CNTs and/orCNHs with a dispersion of the CNTs and/or CNHs.

The invention provides an embodiment of the sheet-like article, in whichthe fiber aggregate is formed of fibers treated with a cationic organicpolymer.

The invention provides an embodiment of the sheet-like article, in whichmain fibers forming the sheet-like article are cellulose fibers.

The invention also provides a method of making the sheet-like article.The method includes the steps of mixing fibers and a dispersion of CNTsand/or CNHs to prepare a mixed dispersion and converting the mixeddispersion into a sheet by a wet papermaking process.

The invention provides an embodiment of the method, in which thedispersion of CNTs and/or CNHs contains a surfactant.

The invention provides an embodiment of the method, which furtherincludes the step of adsorbing a CNT and/or CNH fixing agent to thefibers before the step of mixing the fibers with the dispersion of CNTsand/or CNHs.

The invention provides an embodiment of the method, in which the fixingagent has a polarity opposite to that of the surfactant.

The invention provides an embodiment of the method, in which thesurfactant is anionic, and the fixing agent is cationic.

The invention provides an embodiment of the method, in which the fixingagent is an organic polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-section of a fiber forming the sheet-likearticle of the invention, taken along the longitudinal direction of thefiber.

FIG. 2 is an electron micrograph (×3000) of the sheet-like articleobtained in Example 1.

FIG. 3 is an electron micrograph (×10000) of the sheet-like articleobtained in Example 1.

FIG. 4 is an electron micrograph (×3000) of the sheet-like articleobtained in Comparative Example 3.

FIG. 5 is an electron micrograph (×10000) of the sheet-like articleobtained in Comparative Example 3.

FIG. 6 is a figure equivalent to FIG. 1 of a sheet-like article out ofthe scope of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

One of the essential characteristics of the sheet-like article of theinvention is that CNTs and/or CNHs not only adhere to the surface ofmain fibers forming the sheet-like article but also form a specificnetwork structure covering the surface of the fibers. As used herein,the expression “main fibers (forming the sheet-like article)” isintended to mean the fibers that form at least 51% by mass of thesheet-like article. The electrical conductivity (hereinafter simplyreferred to as “conductivity”) of a CNT or CNH is almost the same asthat of copper. A CNT and a CNH are physically very tough. Therefore, asheet-like article fabricated of fibers covered with a specific networkof CNTs and/or CNHs not only exhibits excellent electricalcharacteristics but have strength and yet flexibility providing ease inhandling, being backed by the main fibers. The invention has thussucceeded in settling the problems with conventional carbonblack-containing sheeting, such as fall-off of paper dust or carbonblack particles. The CNTs and/or CNHs existing on the fibers in the formof a network structure allow efficient conversion of electron kineticenergy, electromagnetic waves, and microwaves to thermal energy, therebyto provide an excellent electromagnetic shielding sheet, microwaveheating element, and sheet heating element.

It is preferred that the specific network structure of CNTs and/or CNHspreferentially cover the main fibers of the sheet-like article. It ispreferred that at least 5%, more preferably 20% or more, of the surfacearea of the main fibers be covered with CNTs and/or CNHs. When thesurface area of the main fibers covered with CNTs and/or CNHs is lessthan 5% based on the total surface area of the main fibers, it isdifficult for CNTs and/or CNHs to form a network structure on the fibersand to provide good electrical characteristics. That is, it is preferredfor obtaining good electrical characteristics that at least 5%, morepreferably 20% or more, of the surface area of the main fibers becovered with CNTs and/or CNHs.

The specific network structure of CNTs and/or CNHs on the fibers is thestructure of a large number of CNTs and/or CNHs adhering to the surfaceof a fiber A (the main fiber) in a uniformly dispersed state withoutagglomeration, as schematically illustrated in FIG. 1. A large number ofCNTs and/or CNHs form a continuous carbon layer (microscopic networkstructure) B with a uniform thickness over substantially the wholelength of the fiber A. A great number of fibers A each having such amicroscopic network structure (i.e., carbon layer B) of CNTs and/or CNHsformed thereon are three dimensionally entangled with each other to forma sheet. Accordingly, the sheet-like article of the invention has, as awhole, a macroscopic network of CNTs and/or CNHs to exhibit excellentelectrical characteristics and heat generating characteristics.

As used herein, the term “network structure of CNTs and/or CNHs” refersnot only to the microscopic network structure continuously formed bymutual connection of CNTs and/or CNHs on the individual main fibers butalso to the macroscopic network structure formed by three dimensionalentanglement of the fibers covered with CNTs and/or CNHs. The CNTsand/or CNHs are thus in continuous contact with each other throughoutthe sheet-like article to provide passageways for electrons, which isconsidered to provide the above discussed very advantageous electricalcharacteristics. However, it is not always necessary for CNTs and/orCNHs to form a carbon layer B with a uniform thickness on a fiber A asillustrated in FIG. 1. The carbon layer B may have a somewhat varyingthickness or may have a surface roughness.

As stated, it is necessary for CNTs and/or CNHs to adhere to the surfaceof individual main fibers forming the sheet-like article of theinvention in a uniformly dispersed state without agglomeration, asschematically illustrated in FIG. 1, thereby to achieve uniformconductivity through the sheet-like article. In the case of, incontrast, a sheet-like article containing fibers having CNTs and/or CNHsadhering thereto in which a large number of CNTs and/or CNHs form anumber of agglomerates C which continuously connect to each other on thesurface of a main fiber A, i.e., the CNTs and/or CNHs are not uniformlydispersed as schematically illustrated in FIG. 6, although electrons areallowed to pass through the agglomerates C, the sheet-like articlehardly provides reliable electrical conduction as a whole. In this case,since no improvement in conductivity is expected, CNTs and/or CNHsshould be used in an increased amount to obtain improved conductivity,which will cause an increase in cost. Consequently, the sheet-likearticle is not preferred. Moreover, the CNTs and/or CNHs added in alargely increased amount will not only cover the fiber surface but fillthe fiber-fiber interstices. As a result, drainage of a wet-laid fiberlayer deteriorates, making sheet formation difficult. Since the CNTsand/or CNHs are not uniformly distributed in the sheet-like article, asschematically shown in FIG. 6, conductivity inside the sheet-likearticle is not uniform, which may lead to such a problem thatelectrification can cause a fire at a portion having high resistance.

Where CNTs or CNHs are discontinuous on the fiber surface, i.e., where alarge number of CNTs and/or CNHs do not form a network structure on thefiber, the CNTs and/or CNHs fail to efficiently convert electromagneticor microwave energy to thermal energy and to exhibit sufficient effectswhen, in particular, the sheet-like article is used as anelectromagnetic shield or a microwave heating element.

The sheet-like article of the invention exhibits conductivity in itsthickness direction, too, because of the network structure of CNTsand/or CNHs extending in not only planar directions (two dimensionaldirections) but also in the thickness direction (three dimensionaldirections) of the sheet-like article. Therefore, the sheet-like articleis suited for use as, for example, a current collector of a batterycell. The network structure of CNTs and/or CNHs extending in thethickness direction also brings about great improvements inelectromagnetic shielding characteristics and the like. In the caseswhere the main fibers are partly covered with CNTs and/or CNHs, theratio of the surface area of the fibers covered with CNTs and/or CNHs tothe total surface area of the main fibers is of importance. In suchcases, a single fiber may have CNTs and/or CNHs adhering to a dividedpart(s) of its surface. The electromagnetic wave shieldingcharacteristics or the characteristics as a microwave heating elementare secured as long as the ratio of the surface area of the main fiberscovered with CNTs and/or CNHs to the total surface area of the mainfibers (hereinafter referred to as “covered surface area ratio”) is 5%or more. With a covered surface area ratio of 20% or more, theperformance as a sheet heating element is markedly improved. With acovered surface area ratio of 50% or more, conductivity is remarkablyimproved, which will allow application to new fields where conductivityis of concern. The covered surface area ratio, a measure of the degreeto which the surface of the main fibers is covered with CNTs and/orCNHs, is a significant parameter; for it relates to the degree of theformation of the network by CNTs and/or CNHs. The degree of theformation of the network by CNTs and/or CNHs is easily demonstrable bydirect observation under an electron microscope

The sheet-like article having the above discussed network structure ofCNTs and/or CNHs is preferably fabricated by (1) a wet papermakingprocess using fibers and a dispersion of the CNTs and/or CNHs(hereinafter also referred to simply as “CNT/CNH dispersion”) or (2)impregnating a fiber aggregate formed without using CNTs and/or CNHswith a CNT/CNH dispersion. In either method, to use a CNT/CNH dispersionis a key point for forming the network structure of CNTs and/or CNHs tomake the sheet-like article.

The method (1) is sheet formation by a wet papermaking process. A wetpapermaking process is a technique commonly used to make paper ornonwoven fabric, which includes dispersing fibers in water, addingnecessary chemicals to the resulting slurry as appropriate, andconverting the slurry into a sheet form using wire cloth. CNTs and CNHsare usually composed solely of surface atoms so that they are attractedto themselves by van der Waals force and eventually exist asagglomerates. If CNTs and CNHs are used to make a sheet in the form ofpowder without being dispersed in a liquid medium, such as water, theywould exists in the resulting sheet in the form of agglomerates asschematically illustrated in FIG. 6 and fail to exhibit their intrinsicperformance discussed above. It is therefore desirable in the inventionthat the sheet-like article be fabricated using a CNT/CNH dispersion.The dispersion preferably contains a surfactant helping CNTs and/or CNHsto be dispersed in a liquid medium.

The manner of dispersing CNTs and/or CNHs, i.e., the method of preparinga CNH/CNHs dispersion is not particularly limited. Examples of usefuldispersions include, but are not limited to, a dispersion prepared bydispersing CNTs and/or CNHs using a surfactant by ultrasonication,bead-milling, or like means; a dispersion prepared by dispersing usingan organic solvent by a physical treatment, such as ultrasonication orbead-milling; a dispersion prepared by making use of a repulsive forcebetween molecules of the same polarity; a dispersion obtained bydispersing CNTs and/or CNHs having a magnetic substance adheringthereto; a dispersion prepared by dispersing CNTs and/or CNHs with theirsurface modified; and a dispersion prepared by a combination of thesetechniques. A CNT/CNH dispersion in water is particularly advantageousto form a network structure of CNTs and/or CNHs on the surface of fibershaving a hydrophilic group, such as cellulose fibers.

Production of the sheet-like article of the invention by a papermakingprocess (the method (1)) will be described taking, for instance, thecase of using cellulose fibers as the main fibers constituting thesheet-like article. The method includes the steps of mixing fibers and aCNT/CNH dispersion to prepare a mixed dispersion and converting themixed dispersion into a sheet form. In order to form the above describedspecific network structure, it is significant to select a combination ofthe manner of dispersing CNTs and/or CNHs and a fixing agent. The fixingagent preferably has a polarity opposite to that of the surfactant.Cellulose fibers are known to be negatively charged when suspended inwater. For example, when CNTs and/or CNHs are dispersed using an anionicsurfactant, a cationic fixing agent is used to fix the CNTs and/or CNHs,whereby the CNTs and/or CNHs show good adhesion to the cellulose fiberswhile forming their network structure probably through the followingmechanism. On being added to the cellulose fibers, the cationic fixingagent adheres to the surface of the cellulose fibers to change thesurface charges of the cellulose fibers to positive. Thus, the CNTsand/or CNHs dispersed with the anionic surfactant are successfullyadsorbed to the surface of the cellulose fibers by an electricalaffinity.

CNTs and CNHs are known to have a very strong tendency to agglomeratedue to van der Waals attraction. In order for CNTs and/or CNHs to beselectively fixed onto the cellulose fibers to form a good networkstructure, it is necessary to prevent them from agglomerating beforethey are fixed on the cellulose fibers. Among the manipulations that maybe used to achieve this is addition of a cationic fixing agent to anaqueous dispersion of cellulose fibers (i.e., a pulp slurry) to fix thecationic fixing agent onto the cellulose fibers followed by addition ofa dispersion of CNTs having the surface charged anionically that isprepared by, for example, using an anionic surfactant as a dispersant.It is preferred that addition of the cationic fixing agent into the pulpslurry be followed by stirring thoroughly. That is to say, it ispreferred that the fixing agent for CNTs and/or CNHs be adsorbed ontothe fibers before the fibers are mixed with the CNT/CNH dispersion.Thus, the cationic fixing agent is uniformly adsorbed on the surface ofthe cellulose fibers, followed by adsorbing the anionically charged CNTsto the adsorbed cationic fixing agent. As a result, the CNTs and/or CNHssuccessfully form a network structure on the surface of the cellulosefibers while being prevented from agglomerating. In the case when thecationic fixing agent is added after the addition of the dispersion intothe pulp slurry, not only adsorption on the cellulose fibers but alsoconsiderable agglomeration of the CNTs and/or CNHs occur.

For use as a fixing agent (cationic fixing agent) organic polymers arepreferable to inorganic fixing agents generally used in papermaking,such as aluminum sulfate, probably for the following reason. It is knownthat an organic polymeric fixing agent is adsorbed on a fiber surface ina train-loop-tail configuration. “Train” segments are directly adsorbedon the surface of a fiber and “loop” or “tail” segments spread out intothe solvent. When a dispersion of anionically charged CNTs and/or CNHsis added to the fiber slurry in which the polymeric fixing agent isadsorbed on the fiber in such a configuration, the loop and tailsegments of the cationic organic polymeric fixing agent are adsorbed onthe negatively charged surface of CNTs to form bridges. This is believedto accelerate adsorption of the CNTs and/or CNHs to the cellulose fibersurface. Accordingly, in order to obtain the fixing effect to themaximum extent, it is preferred that adsorption of the fixing agent onthe cellulose fibers, which may be achieved by adding the fixing agentto an aqueous dispersion of the cellulose fibers, be followed byadsorption of CNTs and/or CNHs on the cellulose fibers, which may beachieved by adding a dispersion of the CNTs and/or CNHs to the aqueousdispersion of the cellulose fibers.

An inorganic fixing agent, like aluminum sulfate, which is generallyused in the field of papermaking, is considered to be fixed by amechanism such that it is adsorbed on the surface of a particle toneutralize the charges and fixed by van der Waals force. In this case,although fixation of the CNTs and/or CNHs to the cellulose fibers andformation of a network structure by the CNTs and/or CNHs are observed,the CNTs and/or CNHs undergo agglomeration due to the very strong vander Waals force between themselves. As a result, the resultingsheet-like article is inferior in performance to the sheet-like articleobtained using an organic polymeric fixing agent.

Formation of a satisfactory network by CNTs and/or CNHs on the surfaceof cellulose fibers is also achieved when the CNTs and/or CNHs aredispersed with a cationic surfactant. In this case, the pulp slurry maypreviously be made anion-rich by adding to the pulp slurry an anionicpolymer, microfibrillated cellulose, or carboxymethyl cellulose, tothereby accelerate fixation of the CNTs and/or CNHs on the cellulosefibers.

The method of making the sheet-like article by the papermaking processis advantageous in that the surfactant used to disperse the CNTs and/orCNHs is removed simultaneously with the sheet formation. When a processother than the wet papermaking process, such as a dry process, isfollowed, the residual surfactant tends to act on the fiber surface tointerfere with the formation of hydrogen bonds between cellulose fibers.Furthermore, the surfactant present on the surface of the CNTs and/orCNHs tends to inhibit the CNTs and/or CNHs from coming into directcontact with each other during the formation of the network structure.By using a wet papermaking process, the CNTs and/or CNHs are allowed toform a network structure in the wet-laid fiber sheet, which is thendewatered to be freed of the surfactant present on the surface of theCNTs and/or CNHs. The above discussed problems are thus settled.

The sheet-like article of the invention is also obtained by the method(2) described above. The method (2) is known as an impregnation method.More specifically, the method (2) includes the steps of making a CNT- orCNH-free sheet or fiber aggregate of the main fibers and soaking thefiber aggregate in a CNT/CNH dispersion to infiltrate the dispersiondeep into the inside of the fiber aggregate, whereby the CNTs and/orCNHs cover the surface of the individual fibers to form a microscopicnetwork while forming a three dimensional, macroscopic networkthroughout the sheet. The manner of covering the individual main fiberswith the CNT and/or CNH network structure is not particularlyrestricted. For example, the sheet or fiber aggregate may be formed ofthe main fibers having been treated with a cationic organic polymer soas to facilitate fixing the CNTs and/or CNHs, and the resulting fiberaggregate is impregnated with a CNT/CNH dispersion. It is also effectiveto use cationic pulp to make main fibers with a cationic surface andconvert the main fibers into a sheet, which is then impregnated with adispersion of CNTs and/or CNHs having been treated to become anionic toform a network structure of the CNTs and/or CNHs on the main fibers.Preferred examples of the CNT/CNH dispersion include a dispersionprepared by dispersing using an organic solvent by a physical treatment,such as ultrasonication or bead-milling and a dispersion prepared bydispersing CNTs and/or CNHs with their surface modified.

When in using a CNT/CNH dispersion containing a surfactant, theresulting sheet-like article may be subjected to post treatment toeliminate the aforementioned interference of the surfactant and toobtain the effects of CNTs and/or CNHs to a sufficient extent. Forexample, the sheet-like article may be washed by immersing in a solventwhich elutes the surfactant to have improved conductivity.

The CNTs and CNHs for use in the invention are not particularly limited.There are various methods for making CNTs, such as CVD, arc discharge,and laser ablation. The effects of the invention do not depend on themethod of making CNTs. There are two types of CNTs: single-walled CNTsthat can be conceptualized by wrapping a single graphene sheet into acylinder and multi-walled CNTs composed of two or more concentricsingle-walled CNTs, either of which is useful. The diameter of the CNTis preferably 1 to 75 nm, more preferably 1 to 50 nm. The length of theCNT is preferably 0.1 μm or longer, more preferably 1 μm or longer. Thediameter of the CNH is preferably 150 nm or less.

The main fibers (main fibers forming the sheet-like article) that can beused in the invention are not particularly limited. Examples of fibersuseful as the main fibers include cellulose fibers, such as wood pulpfibers, e.g., softwood bleached kraft pulp (NBKP), hardwood bleachedkraft pulp (LBKP), softwood bleached sulfite pulp (NBSP), andthermomechanical pulp (TMP), bast fibers, e.g., paper mulberry, orientalpaperbush, and gampi (Diplomorpha sikokiana), non-wood pulp fibers,e.g., straw, bamboo, kenaf, and bagasse, microfibrillated cellulosefibers obtained by treating cellulose fibers, biocellulose fibers, rayonfibers, other surface-treated cellulose fibers, and carboxymethylcellulose fibers; synthetic pulp; synthetic fibers; semisyntheticfibers; inorganic fibers; and metal fibers. These fibers may be usedeither individually or as an appropriate combination thereof. Ifdesired, carbon fibers, activated carbon fibers, conductive fibers, andmetal fibers may be used as well.

To use cellulose fibers as the main fibers is preferred because CNTsand/or CNHs are fixed to the fibers while efficiently forming thenetwork structure for some unknown reason. It is believed that CNTsand/or CNHs are fixed onto the surface of the fibers while forming anetwork structure through the above discussed mechanism becausecellulose fibers are negatively charged due to their hydroxyl groups.Additionally, cellulose fibers have good dispersibility in water owingto the hydrophilic hydroxyl groups, which is advantageous when the sheetis formed by a wet papermaking process.

To use microfibrillated cellulose obtained by disintegrating cellulosefibers under high pressure and shearing force is preferred as providinga denser three dimensional network structure. Microfibrillated celluloseis fine fibers obtained by treating cellulose fibers so as to have anumber average length of 0.05 to 3 mm and a water retention of 200 ml ormore. Using such fine fibers to be covered with CNTs and/or CNHsprovides a denser and more uniform network structure. Microfibrillatedcellulose is very well fixable on cellulose fibers, such as pulp fibers.Therefore, using microfibrillated cellulose or a mixture of cellulosefibers and microfibrillated cellulose to be covered with CNTs and/orCNHs is extremely effective in forming a network structure of CNTsand/or CNHs.

In making the sheet-like article by a wet papermaking process, commonlyemployed papermaking additives may be used within ranges that do notimpair the performance. Examples of such additives include water solublepolymeric strength agents, such as starch, guar gum, and polyvinylalcohol; polymeric strength agents, such as polyacrylamide andvinylamine polymers; wet strength agents, such as melamine and polyamidepolyamine epichlorohydrin; loading materials, such as titanium oxide,calcium carbonate, kaolin, and talc; sizes, such as a rosin size, analkylketene dimer, a sodium alkenyl succinic anhydride, andstyrene-acrylic monomer copolymers; and fixing agents, such as aluminumsulfate, cationized polyacrylamide, and anionized polyacrylamide. Ifdesired, carbon black may be added.

The sheet-like article of the invention, especially the sheet-likearticle containing CNTs is useful as an ultrafine filter. This isconsidered to owe to the ultrafine voids formed by CNTs in which fineparticles are trapped.

The sheet-like article of the invention, particularly the sheet-likearticle containing CNHs is usable as a catalyst carrier with the CNH'sproperty of taking in a substance being taken advantage of.

EXAMPLES

CNTs were dispersed in accordance with the following dispersing methods.

Dispersing Method 1:

CNTs were kneaded with sodium dodecylbenzenesulfonate as a dispersant ina mortar for 20 minutes. The mixture was treated in an ultrasonic bathoperating at a frequency of 38 kHz and a power of 120 W for 2 hours toprovide a CNT dispersion of anionic surfactant type.

Dispersing Method 2:

A CNT dispersion of cationic surfactant type was prepared in the samemanner as dispersing method 1, except for usingdododecyldimethylammonium bromide as a dispersant.

Dispersing Method 3:

CNTs were kneaded in a mortar for 20 minutes and then dispersed inacetone to give a CNT solid content of 2 mass %. The dispersion wasprocessed with ultrasonic waves at 100 W for 20 minutes to give a CNTdispersion of physical treatment type.

Example 1

Fifty percent by mass of hardwood bleached kraft pulp and 50 mass % ofsoftwood bleached kraft pulp were dispersed in water and beaten in adouble disc refiner to prepare a pulp slurry having a CSF freeness of350 ml. Cationic starch (Neotack L-1, from Nihon Syokuhin Kako Co.,Ltd.) was mixed therein in an amount of 2 mass % relative to the pulpmass. The CNT dispersion of anionic surfactant type prepared by thedispersing method 1 was then mixed therein to give a slurry having a CNTsolid content of 5 mass % relative to the pulp mass. The slurry wasconverted into a sheet-like article having a grammage of 100 g/m² by awet papermaking process using a wire cloth.

Example 2

A sheet-like article having a grammage of 100 g/m² was made in the samemanner as in Example 1, except for replacing the CNT dispersion ofanionic surfactant type obtained by the dispersing method 1 with the CNTdispersion of cationic surfactant type obtained by the dispersing method2 and using 3 mass % of carboxymethyl cellulose in place of the cationicstarch.

Example 3

A sheet-like article having a grammage of 100 g/m² was made in the samemanner as in Example 1, except for replacing the CNT dispersion ofanionic surfactant type obtained by the dispersing method 1 with the CNTdispersion of physical treatment type obtained by the dispersing method3.

Example 4

A sheet-like article having a grammage of 100 g/m² was made in the samemanner as in Example 1, except for using 2 mass %, relative to the pulpmass, of aluminum sulfate as a fixing agent.

Example 5

Fifty percent by mass of hardwood bleached kraft pulp and 50 mass % ofsoftwood bleached kraft pulp were dispersed in water and beaten in adouble disc refiner to prepare a pulp slurry having a CSF freeness of350 ml. A polyamide epichlorohydrin resin (WS4002, from Seiko PMC Corp.)was mixed therein as a wet strength agent in an amount of 0.5 mass %relative to the pulp mass. The resulting pulp slurry was converted intoa sheet having a grammage of 100 g/m² by a wet papermaking process. Thesheet was impregnated with the CNT dispersion obtained by the dispersingmethod 3 and dried to give a CNT-impregnated sheet-like article. The CNTcontent of the sheet-like article was 1.5% as calculated from the amountof the dispersion infiltrated into the sheet.

A CNT-impregnated sheet-like article was made in the same manner as inExample 5, except that the unimpregnated sheet was impregnated with a 1%solution of cationic starch as a fixing agent and dried. The resultingimpregnated sheet was further impregnated with the CNT dispersion ofanionic surfactant type obtained by the dispersing method 1 to give aCNT-impregnated sheet-like article.

Comparative Example 1

Fifty percent by mass of hardwood bleached kraft pulp and 50 mass % ofsoftwood bleached kraft pulp were dispersed in water and beaten in adouble disc refiner to prepare a pulp slurry having a CSF freeness of350 ml. Two percent by mass of a polyamide epichlorohydrin resin(WS4002, from Seiko PMC Corp.) was mixed therein. The resulting pulpslurry was converted into a sheet having a grammage of 100 g/m² by a wetpapermaking process using a wire cloth.

Comparative Example 2

A sheet-like article was fabricated using cellulose fibers and, as abinder, polyethylene fibers by a dry process. The article was apparentlyfound to have CNTs agglomerated.

Comparative Example 3

Fifty percent by mass of hardwood bleached kraft pulp and 50 mass % ofsoftwood bleached kraft pulp were dispersed in water and beaten in adouble disc refiner to prepare a pulp slurry having a CSF freeness of350 ml. A CNT powder, which was not dispersed in a liquid medium, wasthen mixed therein in an amount of 5 mass % on a solid basis relative tothe pulp mass, and 2 mass % of aluminum sulfate was also added thereto.The resulting slurry was converted into a sheet-like article having agrammage of 100 g/m² by a wet papermaking process using a wire cloth.

Each of the sheet-like articles obtained in Examples 1 to 6 andComparative Examples 1 to 3 was evaluated for performance in terms ofCNT-covered surface area ratio, electromagnetic shielding properties,microwave heating properties, planarly heat generating properties, andconductivity in accordance with the methods described below. The resultsof evaluation are shown in Table 1. The sheet-like article ofComparative Example 2 was not evaluated because of its incontestableinferiority due to the apparent agglomeration of CNTs.

CNT-Covered Surface Area Ratio

The surface of a sample (sheet-like article) was photographed using anelectron microscope (JSM-6360LA from JOEL, Ltd.) at a magnification of3000 times. The ratio of the area covered with CNTs as calculated on theimage was ranked AA to C as follows. Ranks AA, A and B are appraised aspassable. The evaluation was made on randomly chosen 10 points persample.

AA: 20% or moreA: 10% or more and less than 20%B: 5% or more and less than 10%C: less than 5%

Electromagnetic Shielding Properties

Electromagnetic shielding properties of 100 MHz and of 3 GHz weredetermined by placing a sample (sheet-like article) between a micro loopantenna for radiating electromagnetic signals (loop diameter: 5 mm;available from Keycom Corp.) and a micro loop antenna for receivingelectromagnetic signals (magnetic field probe CP-25, from NEC GlassCorp.), each connected to a spectrum analyzer (R3132, from AdvantestCorp.). The electromagnetic shielding properties at 100 MHz and 3 GHzwere expressed in decibel (dB) and ranked AA to C as follows. Samplesranked AA and A are passable.

At 100 MHz:

AA: 15 dB or moreA: 10 dB or more and less than 15 dBB: 5 dB or more and less than 10 dBC: less than 5 dB

At 3 GHz:

AA: 20 dB or moreA: 10 dB or more and less than 20 dBB: 5 dB or more and less than 10 dBC: less than 5 dB

Microwave Heating Properties

A sample (sheet-like article) was microwaved in a domestic microwaveoven at 600 kW for 30 seconds, and the surface temperature of the samplewas measured with a non-contact thermometer. A sample that heated to atemperature 10° C. or more higher than before being microwaved wasranked B. A sample that heated to a temperature 20° C. or more higherthan before being microwaved was ranked A. A sample that did not heat toa temperature 10° C. or more higher than before being microwaved wasranked C. Ranks A and B are passable.

Planarly Heat Generating Properties

A sample (sheet-like article) was placed on a glass plate. A voltage of12 V was applied to the sample for 5 minutes using electrodes spaced 10cm. After the voltage application, the surface temperature of the samplewas measured with a non-contact thermometer in a constant temperatureroom set at 23° C. A sample the surface temperature of which rose by 10°C. or more was ranked B. A sample the surface temperature of which roseby 20° C. or more was ranked A. A sample the surface temperature ofwhich did not rise by 10° C. or more was ranked C. Ranks A and B arepassable.

Conductivity

A sample (sheet-like article) was conditioned under the conditionsdescribed in JIS P8111 for one day. The volume resistivity of theconditioned sample was measured by the four-terminal method inaccordance with JIS K7194 as a measure of conductivity. The measurementwas taken using Lorest MCP-HT450 from Mitsubishi Chemical Analytech Co.,Ltd. at 23° C. and 50% RH. The measured volume resistivity was ranked asfollows. Ranks AA to B are passable.

AA: less than 1×10¹ Ω·cmA: 1×10¹ Ω·cm or more and less than 1×10² Ω·cmB: 1×10² Ω·cm or more and less than 1×10³ Ω·cmc: 1×10³ Ω·cm or more

TABLE 1 Electromagnetic Covered Shielding Microwave Planarly Heat SheetFormation Surface Area Properties Heating Generating Method Ratio 100MHz 3 GHz Properties Properties Conductivity Example 1 wet papermakingAA AA AA A A AA Example 2 wet papermaking AA A AA A A A Example 3 wetpapermaking A A A B B B Example 4 wet papermaking B A A A A A Example 5impregnation B A A B B B Example 6 impregnation A A A A A A Compara. wetpapermaking C C C C C C Example 1 Compara. dry process N.D. N.D. N.D.N.D. N.D. N.D. Example 2 Compara. wet papermaking C C B C C C Example 3

FIGS. 2 and 3 are electron micrographs taken of the surface of thesheet-like article obtained in Example 1. In FIG. 3, the portionindicated by the arrow is a portion covered with CNTs corresponding tothe carbon layer B of FIG. 1. The micrographs of FIGS. 2 and 3 revealthat CNTs adhere to the cellulose fibers, the main fibers fabricatingthe sheet-like article, in a uniformly dispersed state withoutagglomeration thereby covering the cellulose fibers while forming anetwork structure. In particular, FIG. 2 provides visual observation ofthe network structure covering the cellulose fibers at a surface arearatio of 50% or more.

The sheet-like article of Example 1, which was prepared using acombination of an anionic surfactant for CNT dispersion and a cationicfixing agent, had better performance than the product of Example 2,which was prepared using a combination of a cationic surfactant for CNTdispersion and an anionic fixing agent and that of Example 3 preparedusing a CNT dispersion of physical treatment type. This is considered tobe because the above described fixation mechanism of the cationicpolymer promotes covering of cellulose fibers with CNTs.

Electron microscopic observation of the surface of the sheet-likearticle of Example 4, which was prepared using an inorganic fixingagent, revealed that the CNTs adhere to the cellulose fibers in auniformly dispersed state without agglomeration to cover the surface ofthe cellulose fibers while forming a network structure and thatagglomerates of CNTs fill the interstices of the cellulose fibers. Thewet papermaking system of Example 4 took a slightly longer time fordewatering as compared with the systems of Examples 1 through 3. This isprobably because the CNT agglomerates filling the interstices of thecellulose fibers interfere with drainage.

The sheet-like articles of Examples 5 and 6 are both made by theimpregnation method. The sheet-like article of Example 6 generallyexhibits better performance than that of Example 5. This is consideredto be because of good fixation of CNTs to the cellulose fibers, beingaccelerated by treating the cellulose fibers with the cationic polymer.

Containing no CNTs, the sheet-like article of Comparative Example 1showed poor results in various performance characteristics. While thesheet-like article of Comparative Example 2 was not evaluated forperformance, agglomeration of CNTs was noticeable. In addition, many ofthe CNTs fell off the surfaces of the fibers, apparently suggestinginferiority in performance.

FIGS. 4 and 5 are electron micrographs taken of the surface of thesheet-like article obtained in Comparative Example 3. In FIGS. 4 and 5,the portions indicated by the arrows are agglomerates of CNTscorresponding to the agglomerates C of FIG. 6. FIGS. 4 and 5 reveal thatthe CNTs exist in the form of agglomerates, failing to sufficientlycover the surface of the cellulose fibers. The noticeable agglomerationof CNTs observed in Comparative Example 3 is believed to be because theCNTs were added to the dispersion of fibers in powder form (not in adispersed state as in Examples).

INDUSTRIAL APPLICABILITY

The sheet-like article of the invention is fabricated of fibers having alarge number of CNTs and/or CNHs adhering to the constituent fibersthereof in a uniformly dispersed state without agglomeration to build upa network structure on the fibers. Owing to this structure, thesheet-like article exhibits excellent conductivity, electromagneticshielding properties, microwave heating properties, and planarly heatgenerating properties and are suited for use as a conductive sheet, anelectromagnetic shielding sheet, a microwave heating sheet for cooking,an anti-fog sheet for mirror, an electrode, an ultrafine filter, and soforth.

1-13. (canceled)
 14. A sheet-like article comprising fibers and carbonnanotubes and/or carbon nanohorns, the carbon nanotubes and/or carbonnanohorns adhering to the surface of the fibers in a uniformly dispersedstate without agglomeration to form a network structure on the fibers.15. The sheet-like article according to claim 14, being obtained by awet papermaking process using the fibers and a dispersion of the carbonnanotubes and/or carbon nanohorns.
 16. The sheet-like article accordingto claim 14, being obtained by impregnating a fiber aggregate free fromcarbon nanotubes and/or carbon nanohorns with a dispersion of the carbonnanotubes and/or carbon nanohorns.
 17. The sheet-like article accordingto claim 16, wherein the fiber aggregate is formed of fibers treatedwith a cationic organic polymer.
 18. The sheet-like article according toclaim 14, wherein the fibers mainly comprise cellulose fibers.
 19. Amethod of making the sheet-like article according to claim 14,comprising the steps of mixing fibers and a dispersion of carbonnanotubes and/or carbon nanohorns to prepare a mixed dispersion andconverting the mixed dispersion into a sheet form by a wet papermakingprocess.
 20. The method according to claim 19, wherein the dispersion ofcarbon nanotubes and/or carbon nanohorns contains a surfactant.
 21. Themethod according to claim 20, further comprising the step of adsorbing afixing agent for carbon nanotubes and/or carbon nanohorns to the fibersbefore the step of mixing the fibers with the dispersion of carbonnanotubes and/or carbon nanohorns.
 22. The method according to claim 21,wherein the fixing agent has a polarity opposite to that of thesurfactant.
 23. The method according to claim 22, wherein the surfactantis anionic, and the fixing agent is cationic.
 24. The method accordingto claim 23, wherein the fixing agent is an organic polymer.
 25. Asheet-like article obtained by the method according to claim
 19. 26. Thesheet-like article according to claim 15, wherein the fibers mainlycomprise cellulose fibers.
 27. The sheet-like article according to claim16, wherein the fibers mainly comprise cellulose fibers.
 28. Thesheet-like article according to claim 17, wherein the fibers mainlycomprise cellulose fibers.
 29. A sheet-like article obtained by themethod according to claim
 20. 30. A sheet-like article obtained by themethod according to claim
 21. 31. A sheet-like article obtained by themethod according to claim
 22. 32. A sheet-like article obtained by themethod according to claim
 23. 33. A sheet-like article obtained by themethod according to claim 24.